Basic types of modulation

Today vast amounts of information are communicated using radio communications systems. Both analogue radio communications systems and digital or data radio communications links are used.

However, one of the fundamental aspects of any radio communications transmission system is modulation, or the way in which the information is superimposed on the radio carrier.

In order that a steady radio signal or "radio carrier" can carry information, it must be changed or modulated in one way so that the information can be conveyed from one place to another.

There are very many ways in which a radio carrier can be modulated to carry a signal, each having its own advantages and disadvantages. The choice of modulation have a great impact on the radio communications system. Some forms are better suited to one kind of traffic whereas other forms of modulation will be more applicable in other instances. Choosing the correct form of modulation is a key decision in any radio communications system design.

There are three main ways in which a radio communications or RF signal can be modulated:

- Amplitude modulation, AM:  as the name implies, this form of modulation involves modulating the amplitude or intensity of the signal.

Amplitude modulation was the first form of modulation to be used to broadcast sound, and although other forms of modulation are being increasingly used, amplitude modulation is still in widespread use. 

- Frequency modulation, FM: this form of modulation varies the frequency

in line with the modulating signal.

Frequency modulation has the advantage that, as amplitude variations do

not carry any information on the signal, it can be limited within the receiver to remove signal strength variations and noise. As a result, this form of modulation has been used for many applications including high quality analogue sound broadcasting.

- Phase modulation, PM:   as the name indicates, phase modulation varies

the phase of the carrier in line with the modulating signal.

Phase modulation and frequency modulation have many similarities and are linked - one is the differential of the other. However, phase modulation lends itself to data transmissions, and as a result, its use has grown rapidly over recent years.

Each type of modulation has its own advantages and disadvantages, and accordingly they are all used in different radio communications applications.

In addition to the three main basic forms of modulation or modulation techniques, there are many variants of each type. Again, these modulation techniques are used in a variety of applications, some for analogue applications, and others for digital applications.

Angle Modulation

Angle modulation is a name given to forms of modulation that are based on altering the angle or phase of a sinusoidal carrier. Using angle modulation there is no change in the amplitude of the carrier.

The two forms of modulation that fall into the angle modulation category are frequency modulation and phase modulation.

Both types of angle modulation, namely, frequency modulation and phase modulation are linked because frequency is the derivative of phase, i.e. frequency is the rate of change of phase.

Another way of looking at the link between the two types of modulation is that a frequency modulated signal can be generated by first integrating the modulating waveform and then using the result as the input to a phase modulator. Conversely, a phase modulated signal can be generated by first differentiating the modulating signal and then using the result as the input to a frequency modulator.